They’d give grip to tyres in places that get slippery with mud in the winter. Would also be good for making wind breaks for firepits.

If you know anyone with an allotment they’d be really good for excluding weeds – put cardboard underneath to exclude light completely. In fact, if you’re near Leeds I’d take them off your hands for that purpose!

To clean it, let it soak in water for an hour or so and then rinse the dirt out with a shower head, hose or watering can. Be careful as it will be really heavy and quite fragile when wet – preferably don’t move it again until nearly dry.

Alternatively, just keep it dry, hang it over a washing line and beat dust/dry mud out of it like a rug.

Re: garden uses: Don’t forget that although the grass will decompose and eventually rot down, there are probably artificial fibres holding all that grass together which will not. Don’t allow it to decompose in your garden if you don’t want hundreds and hundreds of tiny nylon threads woven thickly into your soil for years and years to come.